The invention is in the field of electronic reproduction technology and is directed to a method and to an apparatus for the determination of the axial position of an engraving element in an electronic engraving machine for engraving print cylinders for rotogravure, and is also directed to an engraving machine with such an apparatus.
When engraving print cylinders in an electronic engraving machine, an engraving element, which comprises, for example, an electromechanical engraving element with an engraving stylus as cutting tool, moves in the axial direction along a rotating print cylinder. The engraving stylus controlled by an engraving control signal cuts a sequence of cups of different depth arranged in an engraving raster into the generated surface of the print cylinder. The engraving control signal is formed by superimposition of an image signal, which represents the gradations to be engraved between xe2x80x9clightxe2x80x9d (white) and xe2x80x9cdarkxe2x80x9d (black), with a periodic raster signal. Whereas the periodic raster signal effects a vibrating lifting motion of the engraving stylus for producing the engraving raster, the image signal values determined the depths of the cups engraved into the generated surface of the print cylinder and, thus, the engraved gradations.
In order to axially position the engraving element before the engraving and move it along the print cylinder in the axial direction during the engraving, the engraving element is driven by a spindle drive that is often designed as a stepping motor drive. The stepping motor is driven by a motor clock sequence, each clock thereof corresponding to a traversed, axial path increment of the engraving element. By counting the clocks of the motor clock sequence with a position counter, thus, the respective axial position of the engraving element can be identified, the engraving element can be displaced onto a defined axial position by counting a predetermined number of clocks.
Before the start of engraving, the position counter of the stepping motor drive must be reset and the respective, axial actual position of the engraving element upon resetxe2x80x94called the zero positionxe2x80x94must be identified, so that the engraving element, proceeding from the identified zero position, can be subsequently shifted to a desired, axial rated position.
U.S. Pat. No. 5,492,057 already discloses a method for determining the axial position of engraving elements in an electronic engraving machine with the assistance of sensors.
U.S. Pat. No. 5,074,690 likewise already discloses that a toothed comb be employed as an absolute value sensor for determining the zero position in a matrix printer.
The traditional method for determining the zero position of an engraving element occurs either with a first light barrier that recognizes a shadowing element called zero flag that is attached to the engraving element or with an absolute value sensor. Particularly given simultaneous engraving with a plurality of engraving elements, the traditional method has the disadvantage that a plurality of zero flags corresponding in number to the plurality of engraving elements must first be optimally precisely adjusted in order to achieve an adequate precision in the determination of the zero position. Over and above this, the employment of a plurality of absolute value sensors is expensive.
It is therefore an object of the present invention to improve a method and an apparatus for determining the axial position of at least one engraving element in an electronic engraving machine for engraving print cylinders for rotogravure as well as an electronic engraving machine having such an apparatus such that a high precision in the position detection is achieved with relatively little expanse and an automatic implementation becomes possible.
According to the present method and apparatus of the present invention for determining an axial position of at least one engraving element in an electronic engraving machine for engraving a print cylinder, an engraving element engraves a series of cups arranged in an engraving raster into a print cylinder, engraved depths of the cups determining gradations to be engraved between light and dark. For planar engraving of the cups, with the engraving element executing a feed motion along the print cylinder that is directed in an axial direction of the print cylinder. A momentary axial position of the engraving element relative to the print cylinder is determined before the engraving. A stationary toothed comb is provided directed in an axial direction of the print cylinder whereby one of tooth faces of each and every tooth represents an axial distance from an axial reference point as a multiple of a toothed division of the toothed comb. The engraving element has an axial reference mark. The engraving element together with its reference mark is displaced from its momentary position onto a closest, relevant toothed face of the toothed comb into the axial position of the engraving element to be determined. An approximate distance of the reference mark from the reference point is measured. The measured approximate distance is compared to the multiple of the toothed division of the toothed comb. An exact distance of the position of the engraving element from the reference point is determined from the comparison.